Gold ores are treated by a variety of processes. All hydrometallurgical gold recovery processes rely on leaching relatively low concentrations of gold from ores using lixiviants, primarily cyanide solutions. Gold recovery from many ores by cyanide leaching is ineffective, with as little as 30 percent of the gold content of the ore being removed. These ores are called refractory ores. Poor gold recovery from refractory ores is typically caused by the gold being occluded in sulfide mineral grains (usually pyrite and arsenopyrite grains) so that the gold cannot react with the cyanide leach solution or by cyanide-dissolved gold being adsorbed by carbonaceous material present in the ore (this phenomenon is known as “preg robbing”). Ores having both problems are known as double refractory ores.
A common method of treating refractory gold ores to render the gold recoverable by cyanidation is by pressure oxidation in autoclaves. Pressure oxidation oxidizes sulfide minerals, rendering the residue non-refractory. The gold is then dissolved by cyanidation and concentrated by adsorption onto activated carbon or resin (either in adsorption columns or in carbon added to the leaching process (known as Carbon-In-Leach (“CIL”), Resin-In-leach (“RIL”), or Carbon-In-Pulp (“CIP”) techniques). The adsorbed gold is eluted from the loaded carbon or resin by stripping with ammonia, nitric acid, caustic and/or steam. The gold is then converted to a solid from the eluate by electrowinning, precipitation and filtration, or cementation.
To reduce process operating and capital costs and/or enable autogenous autoclave operation, it is desirable to concentrate ores by suitable techniques, particularly froth flotation. Froth flotation uses differences in physico-chemical surface properties of particles to float various minerals. After treatment with reagents, such differences in surface properties between the minerals within the flotation pulp are emphasized as either hydrophobic (water repelling)/aerophillic (air attracting) on the one hand or hydrophilic (water attracting)/aerophobic (air repelling) on the other. Air bubbles sparged through the pulp attach to and float hydrophobic particles. Because autogenous autoclave operation commonly requires a sulfide sulfur concentration in the autoclave feed of at least 6.5 wt. %, flotation reagents and conditions are selected to favor flotation of selected gold bearing sulfide minerals.
A conventional flotation circuit for refractory gold ores is shown in FIG. 1. The feed material 100, including gold bearing sulfides, is ground in comminution circuit 104 to provide a comminuted material having a particle size selected to liberate gold and sulfides containing gold. The comminuted material is conditioned with selected reagents (e.g., collectors, frothers, and regulators) and floated in a primary rougher flotation circuit 108 to form a rougher flotation concentrate 116. The primary rougher tails may be floated in a secondary rougher flotation circuit 112 to provide a secondary rougher concentrate 116. The secondary rougher tails are then floated in a first scavenger flotation circuit 120 and the first scavenger tails in an optional second scavenger flotation circuit 124 to provide final scavenger tails 144. The concentrates from the first and second scavenger flotation circuits 120 and 124 are floated in first and (optional) second scavenger cleaner flotation circuits 128 and 136 to provide scavenger cleaner concentrate 132 and scavenger cleaner tails 140.
Flotation of refractory gold-bearing sulfide ores can be challenging for a number of reasons. Gold bearing sulfide ores are commonly fine grained, thereby requiring a finer grind for liberation. Finer grinding can lead to undesirable results. Fine grinding can cause oxidation of sulfides, particularly arsenopyrite, resulting in poor flotation performance. Finer grinding can generate ultrafines (known as slimes), which are difficult to float selectively. Gold-bearing sulfides, such as arsenopyrite, have a high specific gravity and can accentuate the sliming problems during classification. Finer grinding can liberate carbonaceous matter and other deleterious gangue minerals, thereby resulting in significantly higher reagent consumption and inhibiting sulfide mineral flotation. Carbonates present in many gold ores can cause gypsum formation and precipitation on the mineral surfaces of the particles. Gypsum deposits can adversely impact the ability of collectors to adsorb to the surfaces. Flotation selectivity of ultra fine mineral species can be challenging.